This invention relates to a continuous drive type Michelson interferometer for use in Fourier spectroscopy, and more particularly, to means for controlling constant the movement of a movable mirror of such a Michelson interferometer.
One of well-known processes for spectroscopic analysis of various specimens is Fourier spectroscopy using a Michelson interferometer. The principle of the Michelson interferometer will be briefly described with reference to FIG. 1. The Michelson interferometer is an interferometer in which light from a source 1 strikes a beam splitter 2 at an angle of 45.degree. where the light is divided into two beams, that is, light beams reflected and transmitted by the splitter 2. The reflected beam is directed to a stationary mirror 3 and reflected thereby, the transmitted beam is directed to a movable mirror 4 and reflected thereby, and the beams are then recombined by the beam splitter 2 to enter a detector 5 where interference fringes are formed due to the difference between the beam paths. In carrying out Fourier spectroscopy, the movable mirror is continuously moved at a predetermined constant speed in a direction toward and away from the beam splitter. The detector responds to the continuously varying interference fringes to produce an AC signal (Interferogram) which is Fourier transformed by means of a computer. Provided that the movable mirror is moved at a predetermined constant speed v, the beam path difference x becomes a function of time t, that is, x=2vt, and therefore, the output of the detector also becomes a function of time. A light component having a wave number .nu. is converted into an AC signal having a frequency f=2v.nu. Hz.
It is to be noted that the sample to be analyzed is generally located in that portion of a beam path along which a light beam travels from the beam splitter 2 to the photodetector 5. In order to compare the sample with the reference material, the beam path from the beam splitter 2 to the photodetector 5 is generally divided into two beam paths, and a cell filled with the sample is inserted in one beam path and a cell filled with the reference material is inserted in the other beam path.
The above-described continuous mirror drive is required to maintain the moving speed of the movable mirror accurately constant. More specifically, it is required for spectral analysis to move the movable mirror at the predetermined constant speed with a precision of the order of wave length unit, that is, a few micronmeters (.mu.m). To this end, conventional continuous drive type Michelson interferometers generally use electromagnetic drive means, like voice coils often used in speakers, in order to drive the movable mirror to thereby facilitate electrical precise control, and a pneumatic bearing in order to support and guide the movable mirror during movement. For the purpose of controlling the moving speed of the movable mirror, an auxiliary Michelson interferometer is often built in the main Michelson interferometer by forming an auxiliary mirror on the rear side of the movable mirror (that is, that surface of the movable mirror which is opposed to the main reflective surface facing the beam splitter of the main Michelson interferometer). A small sized Michelson interferometer is constructed using this auxiliary mirror as a movable mirror. The auxiliary interferometer uses a light source capable of emitting a light beam of a particular wavelength, that is, a He-Ne laser beam. A detector of the auxiliary interferometer detects the interference fringes of light of the particular wavelength formed during movement of the movable mirror, to thereby produce an AC signal whose frequency corresponds to the moving speed of the movable mirror. This correspondence of the signal frequency to the moving speed allows the mirror driving means to be controlled in a feedback manner such that the voltage converted from the frequency of the AC signal may have the same magnitude as the preset voltage.
The prior art continuous drive type Michelson interferometers as mentioned above, however, have the following problems because pneumatic bearings are used to support and guide the movable mirror. The pneumatic bearings are relatively large-sized and require the use of a compressor which adds to the cost. Other disadvantages of the pneumatic bearings are that they are less durable, that foreign matters into the lubricating space can prevent smooth movement, and that bearing elements are considerably difficult to machine.
It will be envisioned to use mechanical support means such as roller bearings for the purpose of supporting and guiding the movable mirror. The use of such mechanical support means will eliminate the problems associated with pneumatic bearings and is less costly. The mechanical support means such as roller bearings, however, have the drawback that they cannot maintain the mirror moving speed constant under load fluctuation, resulting in speed variations which induce errors. This drawback is not completely eliminated by adding an auxiliary Michelson interferometer such that the moving speed of the movable mirror is controlled in accordance with the AC signal of the detector of the auxiliary interferometer. It has been recognized difficult to actually use mechanical support means like roller bearings to support and guide the movable mirror during movement.
It is, therefore, an object of the present invention to provide a continuous drive type Michelson interferometer system which can control the moving speed of the movable mirror constant with significantly higher precision than in the prior art and allows mechanical support means such as a roller bearing to support and guide the movable mirror during movement, thereby reducing the cost and increasing the effective life of the interferometer as compared with prior art ones.